In order to indicate a rotation of a body, subsequently generally designated as shaft without limiting the invention thereto, it suffices in the simplest case to use an indicator disposed torque proof at a shaft, wherein the indicator indicates the rotational position.
As soon as the shaft rotates by more than one full revolution, the total amount of rotation cannot be read from such a simple indicator anymore.
An indicator of this type can be mechanically coupled with the shaft, but it can also be coupled touch free, e.g. magnetically coupled.
A typical known solution is disposing a permanent magnet eccentrically on the shaft and detecting its rotational position touch free through a magnetic field sensitive sensor, in particular a multi-Hall assembly.
This multi-Hall assembly, which is typically configured as a chip, measures the position of a magnetic field, thus of the field lines as a vector in its measuring plane, typically in the chip plane.
This has the advantage that the absolute field strength and also the direction of the magnetic field transversal to the measuring plane hardly influence the measuring result.
In order to indicate rotations over more than one revolution (so-called multi-turn), there are several options.
Between a shaft and an indicator, there is a transmission ratio that is large enough, so that the maximum number of revolutions can be indicated with a single indicator.
This assembly certainly has the disadvantage that small rotation angle changes of the shaft are reflected by the indicator only in a comparatively imprecise manner.
There is also the option to couple the shaft with two indicators, e.g. to couple the two indicators with different stages of a step-down transmission disposed there between.
Then, however, the first indicator can essentially only represent the completed revolutions performed, wherein the second indicator represents the fractions of a revolution.
On the one hand side, an increasing number of transmission stages is required for an increasing number of revolutions and the mechanical complexity is increased.
Since typically instead of a simple indicator hand, an angle sensor element is required, which supplies an electrical signal, e.g. the magnetic field sensitive angle sensor element described supra, a configuration of this type furthermore has the disadvantage that the angle sensor elements used for the different transmission stages must have different configurations and cannot be identical components as described e.g. in EP 10 76 809.
Another option is to select the transmission ratio of the at least one transmission stage of the step-down transmission rather low, e.g. when using a gear transmission to select it as N:N+/−1, where N is the number of teeth on the interacting gears.
Then, however, the angle sensor element of the shaft which is not stepped down indicates the rotational position within a complete revolution; however, the angle sensor element of the next stepped down transmission stage indicates a rotational position, which deviates from the number of completed revolutions.
However, it is known from DE 10 2005 035 107 and also from DE 198 21 467 A1, how the number of complete revolutions can be computed from a super position of the two cyclically repeating signals of the two sensor elements through respective processing electronics, wherein the signals, however, differ through a respective transmission ratio with respect to their cycle lengths.
With this solution principle, however, in particular where it shall preferably operate for the recited reasons touch free through magnetic field sensitive sensors, however, numerous problems occur which cannot be solved with the prior art solutions recited supra.
On the one hand side, the goal for an angle sensor like for any sensor is to build the sensor as small as possible, since it shall occupy the least installation space possible in a surrounding application.
Since the magnets disposed on the particular transmission stages, in principle, however, can also influence the sensor associated with the respective other magnet, when the magnets are moved closer and closer to one another, this limit condition would rather indicate a rather large offset of the two magnet assembly in radial or in axial direction.
In case the magnet assemblies and transmission stages shall be very small with a small distance in between, in practical applications with a distance below 1 cm, in order to save installation space, the magnetic field generated by the magnet assembly has to be influenced in a controlled manner, wherein a differentiation has to be made between the useful field to be measured by the angle sensor element of the sensor and the scatter field which is insignificant for the measurement.
For a magnet assembly, which is configured not only from one magnet, but from two magnets, disposed opposite to one another with respect to the rotation axis, e.g. with their pole axis parallel to the rotation axis with opposite pole directions, the useful field is the one half of the annular inner portion of the magnetic field, while the scatter field is formed by the two respective field line circles disposed on the outsides of the magnet and the rest of the inner magnet field ring.
In this context, the shielding of the magnetic field sensitive sensor and/or of the magnet assembly on the backside through simple magnetically insulating shielding plates made from a magnetizable material is already known. Such simple shielding plates, however, also deflect the useful field in an undesirable manner, and thus degrade the measuring precision of a sensor of this type.